Method and device for dynamic adjustment of the roll gap in a roll stand of a mill train having multiple stands

ABSTRACT

A method of dynamic adjustment of the roll gap in a roll stand of a mill train having multiple stands for rolling a strip, with a strip stock, i.e., a loop, between two roll stands being adjusted or limited by a loop or a strip stock control, with the dynamics in adjustment of the roll gap being limited as a function of state variables of the mill train, in particular state variables of the loop or strip stock control.

This application is a 371 of PCT/DE97/02473 filed Oct. 28, 1997.

BACKGROUND INFORMATION

The present invention relates to a method and a device for dynamicadjustment of the roll gap in a roll stand of a mill train havingmultiple stands.

FIELD OF THE INVENTION

For strip rolling in a mill train as described, for example, U.S. Pat.No. 3,170,344 and an artivular by S. Duysters et al. entitled “DynamicModeling Of The Finishing Train of Hoogovens' Hot Strip Mill AndOptimization of Thickness Control Parameter,” Journal A, vol. 31, no. 4,Dec. 1, 1990, pp 8-15, describe that for strip rolling in a mill train,in hot wide strip finishing strip mill and optimization of thicknesscontrol parameters” Journal A, vol. 31, no. 4, Dec. 1, 1990, pages 8through 15, XP00017, in particular in hot wide strip finishing mills,there is on average a greater deviation in thickness at the head offirst strips and conversion strips due to technological factors. On thebasis of the thickness measurement downstream from the finishing train,the object of thickness control is to adjust the deviating stripthickness to the original setpoint or an advantageously redisposedsetpoint as quickly as possible. There is a disturbance in mass flow,hereinafter referred to as a mass flow disturbance of the first type,due to the required control action at the screw-down position, e.g., ofthe last stand. This disturbance is even greater, the more quickly thethickness error is eliminated. However, there is a different upper limitfor each strip for the allowed mass flow disturbance and thus for thethickness control rate, and this limit is determined by the correctionpotential available in loop control for the steepness of disturbance,which depends on the rise error response of the control system.

In principle, the screw-down system, whether hydraulic gap control (HGC)or motor-driven gap control (MGC), has a higher dynamic response thanthe main drives, so it is possible for the screw-down system to generatemass flow disturbances whose correction would exceed the dynamicresponse of the controlling element of the loop control, and thus theycan no longer in principle be corrected by the loop control Therefore, .. . the desired rate of correction of thickness errors and the allowedmass flow disturbances with respect to the loop control.

In addition to the greater mass flow disturbances due to the thicknesscontrol, i.e., mass flow disturbances of the first type which arerelevant only at the head of the strip, substantial mass flowdisturbances can also occur under certain conditions due to divergenceeffects of the AGC algorithm (AGC=automatic gauge control; a function ofload roll gap disturbance compensation based on roll separating force)which is based on positive feedback. These disturbances, hereinafterreferred to as mass flow disturbances of the second type, may occur witha distribution over the entire strip due to divergence effects. The AGCalgorithm is based in principle on a positive feedback response in themanner of a geometric series. The series normally converges so that thescrew-down position merges into a new steady-state end value after aload roll gap disturbance. In the event of the unfavorable mechanicalcondition whereby the screw-down and roll separating force measurementin the stand are arranged together (e.g., top-top) instead of oppositeone another (e.g., top-bottom), the series may diverge for the durationof frictional grip occurring in the stand window, so the AGC algorithmthen diverges until frictional grip is broken, resulting in considerablemass flow disturbances of the second type.

To prevent great mass flow disturbances of the first type, the thicknesscontrol is usually adjusted relatively slowly to always be on the safeside. The allowed mass flow disturbance is different with each strip andeach roll stand, depending on the roll pass schedule, i.e., it dependson numerous influencing factors, but its size is unknown, so aconsiderable portion of the control rate which is actually possible withmost strips is not utilized in this compromise.

To limit the effects of mass flow disturbances of the second type whichare possible with certain constellations, only an AGC undercompensationfactor of considerably less than one has proven feasible there so far.The resulting loss of efficiency in correcting skid marks, i.e., coldspots in the strip, would have to be accepted with this compromise.

SUMMARY

An object of the present invention is to provide a method and a devicewhich avoids the above-mentioned disadvantages of known methods anddevices.

The object is achieved according to the present invention by providing amethod and device which dynamically adjusts the roll gap in a roll standof a mill train having multiple stands for rolling a strip, with a stripsupply, i.e., a loop, between two roll stands being adjusted and limitedby loop or strip supply control, the dynamic response of the adjustmentof the roll gap being limited as a function of state variables of theloop or strip stock control. Such a method has proven especiallysuitable in avoiding the above-mentioned disadvantages. The method ofachieving this object according to the present invention is alsosuperior to a strict limitation as a function of state variables of themill train as described in European Patent No. 680,021 A1, for example,or a limitation described in German Patent No. 195 11 267 C1. Dynamicresponse in setting the roll gap is advantageously limited by limitingthe rate at which the roll gap is adjusted. It has proven advantageouswhen reducing the roll nip in this way to perform the rate limitationindependently of the rate limitation when increasing the roll gap.

The roll gap of roll stands in a mill train having multiple stands isusually adjusted by strip thickness controllers which determine the rollgap setpoint as a function of the system deviation of the thicknesscontroller, i.e., the difference between a predetermined required stripthickness and the actual strip thickness. The size of the systemdeviation before entering the strip thickness controller isadvantageously limited as a function of state variables of the loop orstrip stock control.

In another advantageous embodiment of the present invention, the rollgap is adjusted according to a roll gap setpoint by a hydraulic gapcontrol (HGC), with the rate of change of the additional HGC setpointbeing limited according to FIG. 1 or an equivalent parameter. In analternative advantageous embodiment of the present invention, the rollgap is adjusted by a motor-driven gap control (MGC), with the equivalentthickness system deviation being limited according to FIG. 2 or anequivalent parameter. Limitation of the additional HGC setpoint inhydraulic gap control and limitation of the equivalent thickness systemdeviation with motor-driven gap control have both proven to beespecially suitable for limiting the rate in the adjustment of the rollgap.

In another advantageous embodiment of the present invention, the dynamicresponse and the rate of adjustment of the roll gap are limited as afunction of at least one of the following parameters:

strip stock upstream from the roll stand or an equivalent parameter;

strip stock downstream from the roll stand or an equivalent parameter;

system deviation of the loop or strip stock control, i.e., thedifference between the setpoint and the actual value of the loop heightor the strip stock, for the loop or the strip supply upstream from theroll stand;

system deviation of the loop or strip stock control for the loop heightor the strip stock downstream from the roll stand

time derivative of the strip stock upstream from the roll stand;

time derivative of the; strip stock downstream from the

roll stand time derivative of the system deviation of the loop or stripstock control for the loop height or the strip stock upstream from theroll stand;

time derivative of the system deviation of the loop or the strip stockcontrol for the loop height or the strip stock downstream from the rollstand;

roll separating force;

motor current of the roll stand drive;

rpm of the roll stand drive;

torque of the roll stand drive;

It has proven especially advantageous to limit the rate of adjustmentwhen increasing the roll gap as a function of the system deviation,i.e., the difference between the setpoint and the actual value, of theloop height or of the strip stock upstream and downstream from the rollstand.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a limitation of the roll gap adjusting rate with hydraulicgap control according to the present invention.

FIG. 2 shows a limitation of the roll gap adjusting rate withmotor-driven gap control according to the present invention.

FIG. 3 shows a diagram for definition of the matching function accordingto the present invention.

FIG. 4 shows the principle of hydraulic gap control according to thepresent invention.

FIG. 5 shows the principle of motor-driven gap control according to thepresent invention.

DETAILED DESCRIPTION

FIGS. 1 and 2 show two advantageous embodiments for limiting the statevariable with hydraulic gap control (HGC) and motor-driven gap control(MGC). In the latter case, a suitable procedure may be implementedseparately for thickness control.

FIGS. 1 and 2 show reference numbers 1, 2, 3, 4, 5 and 6 for matchingfunctions, reference numbers 7, 8 and 9 for roll stands x⁻¹, x and x₊₁in a mill train having multiple stands, reference numbers 10 and 11 forminimum forming units and reference numbers 12 and 13 for multiplicationpoints. Matching functions 1, 2, 3, 4, 5 and 6 and minimum forming units10, 11 are part of a fuzzy system for forming reduction factors k_(auf)and k_(zu) for the rate limitation when increasing and reducing the rollgap. Loop lifters 20 and 21 are arranged between roll stands 7, 8, 9,for maintaining a predetermined tension in rolled strip 22. Depending onthe condition in the mill train, strip supply s, which is equivalent tothe screw-down angle of the loop lifter, is increased or decreased. InFIGS. 1 and 2, s_(x−1) denotes the strip stock between roll stands 7 and8, i.e., upsteam from roll stand x, and s_(x) denotes the strip stockbetween roll stands 8 and 9, i.e., downstream from roll stand x.s*_(x−1) denotes the required strip stock between roll stand 7 and rollstand 8 and s*_(x) denotes the required strip stock between roll stand 8and roll stand 9. According to FIGS. 1 and 2, difference Δs_(x−1) anddifference Δs_(x) are formed between strip stock setpoint s*_(x−1) ors*_(x) and strip stock actual value s_(x−1) or s_(x). This differenceΔs_(x−1), Δs_(x) can be used as a system deviation for controlling looplifters 20 and 21, for example. Furthermore, difference Δs_(x−1) mergesinto matching functions 1 and 2, and difference Δs_(x) merges intomatching functions 3 and 4.

Output variables of the matching functions are matching m₁, m₂, m₃, m₄.Furthermore, matches m_(i) and m_(F) are formed from armature current ofthe main drive i_(A,x) and roll separating force F_(w,x) on roll stand xby using matching functions 5 and 6. Matches m_(i), m_(F), m₂ and m₃ aresent to minimum forming unit 10, and matches m₁ and m₄ are sent tominimum forming unit 11. Minimum forming units 10 and 11 function asdefuzzifiers.

Matching function 6 with which roll separating force F_(w,x) merges isan optional additional extension. In this way, the function of overloadprotection can be implemented especially advantageously.

Matching function 5 with which main drive current i_(A,x) merges is alsoan optional extension. By including this matching function 5, inparticular the load redistribution performed regularly betweensuccessive stands by manual screw-down interventions at limit dimensionswith regard to achieving main drive current limits can be securedautomatically.

Output variables of fuzzy logic and thus of minimum forming units 10 and11 are the two reduction factors k_(auf) and k_(zu) which are smallerthan or equal to one and with which an upper and a lower variablelimitation, acting on an intermediate variable which has an influence onthe correcting rate of the screw-down system and is standardized to therate of change in thickness, are adjusted according to worst casescenarios based on positive feedback so that the intermediate variableis adapted to mass flow changes which are evidently yet to beimplemented by the loop control in the sense of flanking measures. Suchan intermediate variable which influences the correcting rate of thescrew-down system may be, for example, additional AGC setpointh_({overscore (B)}) with two-loop AGC or additional AGC setpointds*_(H,A) for HGC, as shown in core structure 15 in HGC in FIG. 1. Thestate variable influencing the correcting rate of the screw-down systemmay also be, for example, equivalent thickness system deviationΔ{circumflex over (h)}, as shown in core structure 14 in MGC in FIG. 2.

The basic consideration in designing the matching functions is that thedirection of effect of screw-down changes on strip supplies of adjacentloops may have an improving or exacerbating trend, depending on the plusor minus sign of the strip stock control deviation. In the case of animproving trend, there is no reason for intervention; from thestandpoint of that loop, the reduction factor may remain at one, i.e.,without any effect. If it is an exacerbating trend, the rate of travelallowed instantaneously is decreased in the corresponding direction.However, this does not mean that the limitation is also reached here,because AGC and thickness control initially function independently ofthis measure. To this extent, loop-controlled dynamic limitation byusing the limits, i.e., reduction factors K_(auf) and K_(zu), is only aflanking measure. By reducing the rate of travel, the loop causing thiscreates the prerequisite for rapid correction of strip stock.

FIG. 3 shows a possible advantageous method of defining the matchingfunctions from FIGS. 1 and 2. The following indices are used in FIG. 3:

u=lower (loop too low) Δs>0

o=upper (loop too high) Δs<0

Δs=strip supply system deviation Δs=s*_(x)−s_(x)

The maximum value for a positive Δs is s* because the minimum value fors is zero (strip tight in the pass line, i.e., zero stock).

Negative values of Δs may achieve much higher absolute values than s*,so the criteria of the flanking measures need not be as strict here.Therefore, the matching function is projecting to the left, i.e., thezero pass of the slope is not limited to at most s*, but instead it canbe extended to (−2)·s*, for example, as assumed in the figure.

Zero passes for the specific angular projection are as follows:

Δs>0: f_(u)·s*, 0.5≦f_(u)≦1.0

Δs<0: −f_(o)·s*, 0.5≦f_(o)≦2.0

The ordinate is at 1.0 in each case. The linear equations forprogramming the fuzzy logic section by section are thus: $\begin{matrix}{K_{i,u} = {{1,0} - {\frac{1}{f_{u} \cdot s^{*}} \cdot {\Delta s}}}} & {{for}\quad m_{1}\quad {and}\quad m_{3}} \\{K_{i,o} = {{1,0} + {\frac{1}{f_{o} \cdot s^{*}} \cdot {\Delta s}}}} & {{for}\quad m_{2}\quad {and}\quad m_{4}}\end{matrix}$

FIG. 4 shows the principle of hydraulic gap control for adjusting a rollgap h in a roll stand 31. Roll separating force F is measured first andthen sent to a load roll gap disturbance compensation circuit 30 (AGC).The output variable of this circuit 30 is ds*_(H,A). Sum s*_(H) fromthis additional AGC setpoint dS*_(H,A), the setpoint determined by thestrip thickness control for roll gap ds*_(D) and basic screw-downposition setpoint s*_(H,O) is the input variable for HGC positioncontrol circuit 32 which adjusts screw-down position s_(H) for rollstand 31. In addition to the limitation on rate of increase or change inthe additional AGC setpoint according to FIG. 1, the rates of increaseor change in ds*_(H,A), ds*_(D) or the sum of ds*_(H,A), ds*_(D) ands*_(H,O) can also be limited.

FIG. 5 shows a schematic diagram of a motor-driven gap control foradjusting roll gap h in a roll stand 34. Roll separating force F ismeasured in roll stand 34 and sent together with basic screw-downposition setpoint S*_(M,O) and an additional setpoint ds*_(D) for rollgap h determined by a strip thickness control to a motor-driven gapcontrol 33. The output variable of motor-driven gap control 33 is ascrew-down rate setpoint s·* M, which is the input variable of aregulated motor 35. The output variable of the regulated motor is ascrew-down position s_(M).

What is claimed is:
 1. A method of dynamically adjusting a roll gap in a roll stand of a mill train having multiple stands for rolling a strip, comprising the steps of: adjusting a loop between two of the roll stands by a loop control; dynamically adjusting the roll gap; and dynamically limiting dynamics of the dynamic adjustment of the roll gap as a real-time function of internal state variables of the loop control.
 2. The method according to claim 1, further comprising the step of: limiting a rate at which the roll gap is adjusted as a function of the state variables of the loop control.
 3. The method according to claim 1, further comprising the step of: limiting a rate at which the roll gap is reduced independently of a rate at which the roll gap is increased.
 4. The method according to claim 1, further comprising the steps of: adjusting the roll gap by a strip thickness controller, the roll gap being adjusted by controlling a roll gap setpoint as a function of a system deviation of the strip thickness controller, the system deviation being a difference between a predetermined required strip thickness and an actual strip thickness; and limiting a size of the system deviation before entering the strip thickness controller as a real-time function of the state variables of the loop control.
 5. The method according to claim 1, wherein the adjusting the roll gap step includes the step of adjusting the roll gap by hydraulic gap control, a rate of change of the hydraulic gap control setpoint being limited to a predetermined rate.
 6. The method according to claim 1, wherein the adjusting the roll gap step includes the step of adjusting the roll gap by motor-drive gap control, an equivalent thickness system deviation being limited to a predetermined deviation.
 7. The method according to claim 1, further comprising the step of: determining limit values to limit one of i) the dynamics of the adjustment of the roll gap, and ii) a rate of adjustment of the roll gap, the limit values being determined by one of fuzzy techniques and mapping techniques.
 8. The method according to claim 1, further comprising the step of: determining limit values to limit one of i) the dynamics of the adjustment of the roll gap, and ii) a rate of adjustment of the roll gap, the limit values being determined by neural networks.
 9. The method according to claim 1, further comprising the step of: determining limit values to limit one of i) the dynamics of the adjustment of the roll gap, and ii) a rate of adjustment of the roll gap, the limit values being determined as a function of at least one of: a strip stock upstream from the roll stand; a strip stock downstream from the roll stand; a system deviation of a loop control for a loop upstream from the roll stand; a system deviation of a loop control for a loop height downstream from the roll stand; a system deviation of the strip stock downstream from the roll stand; a time derivative of the strip stock upstream from the roll stand; a time derivative of the strip stock downstream from the roll stand; a time derivative of a system deviation of the loop control for a loop height upstream from the roll stand; a time derivative of the system deviation of the loop control for a loop height downstream from the roll stand; a roll separating force; a motor current of a roll stand drive; and a torque of the roll stand drive.
 10. The method according to claim 9, further comprising the step of: limiting a rate of adjustment in increasing the roll gap as a real-time function of the system deviation of the loop height upstream from the roll stand and the system deviation of the loop height downstream from the roll stand.
 11. The method according to claim 9, further comprising the step of: limiting a rate of adjustment in reducing the roll gap as a real-time function of one of i) the system deviation in the loop height upstream from the roll stand and the system deviation in the loop height downstream from the roll stand, and ii) the system deviation in the strip stock upstream from the roll stand and the system deviation in the strip stock downstream from the roll stand, the rate of adjustment in reducing the rolling gap further being limited as a function of the motor current of the roll stand and a roll separating force.
 12. A device for dynamically adjusting a roll gap in a roll stand of a mill train having multiple stands for rolling a strip, comprising: a loop control adjusting a loop between two of the stands and dynamically adjusting the roll gap, dynamics of the dynamic adjustment of the roll gap being dynamically limited as a real-time function of state variables of the loop control.
 13. A method of dynamically adjusting a roll gap in a roll stand of a mill train having multiple stands for rolling a strip, comprising the steps of: determining an actual strip thickness; determining a difference between a predetermined strip thickness and the actual strip thickness; and adjusting the roll gap as a function of the determined difference, a rate of adjustment of the roll gap being limited as a function of internal state variables of a loop control.
 14. The method according to claim 13, wherein a rate at which the roll gap is reduced in independent of a rate what which the roll gap is increased.
 15. A method of dynamically adjusting a roll gap in a roll stand of a mill train having multiple stands for rolling a strip, comprising the steps of: adjusting a loop between two of the roll stands by a loop control; dynamically adjusting the roll gap; and dynamically adjusting limitations of the dynamic adjustment of the roll gap as a real-time function of internal state variables of the loop control. 